Improved method of fractionally distilling liquids



May 1, 1962 c. F. BETHEA ETAL 3,032,478

IMPROVED METHOD OF FRACTIONALLY DISTILLING LIQUIDS Filed July 15. 1955 4 Sheets-Sheet 1 51 54 OVERHEAD l l l l PRODUCT l so I FEED

. 8 l BOTTOMS F/G. FIG. .2

5| OVERHEAD PRODUCT OVERHEAD PRODUCT FEED IN VEN TORS C. F. BETH EA BY L.W. POLLOCK BOTTOMS E 2 FIG. 5

ATTORNEYS May 1-, 1962 C. F. BETHEA ETAL IMPROVED METHOD OF FRACTIONALLY DISTILLING LIQUIDS Filed July 15. 1955 4 Sheets-Sheet 2 REFLUX TRAY ACTUATING 29 MEANS OVERHEAD T [8 F M TO CONDENSER AND 3 VACUUM SYSTEM is 3,570n-HEPTANE P1 ['1 23 T A 1 ,760 wA ER 1 OVERHEAD MM o F PRESSURE FEED FEED 53 3,570 n-HEPTANE I E 30,725 POLYBUTADIENE I .l I U VACUUM 1 2s FRACTIONATOR 1 STEAM 2s PERFORATED TRAYS 23,760 WATER BOTTOMS (IO TRAYS REQUIRED) 350 F BOTTOMS 9 30,725 POLYBUTADIENE INVENTORS C.F. BETHEA QUANTITIES lN POUNDS BY .w. POLLOCK PER DAY M M F FIG. /0

ATTORNEYS May 1, 1962 c. F. BETHEA ETAL 3,032,478

IMPROVED METHOD OF FRACTIONALLY DISTILLING LIQUIDS Filed July 15. 1955 4 Sheets-Sheet 5 TO CONDENSER AND OVERHEAD vAcuuM sYsTEM 1 42,490 WATER [6 MM 70,037 ETHANOL PREssuRE 75 F FEED 52,|53 SODIUM ETHYL I XANTHATE 9|,95s wATER 70,752 ETHANOL 2,798 IMPURITIES p REBOILER vAcuuM 90 F FRACTIONATOR 25 TRAYS 52,l63 SODIUM ETHYL 65 TRAYS REQUIRED) BOTTOMS XANTHATE 49,455 wATER QUANTITIES IN POUNDS PER DAY 725 ETHANOL F/G 2,798 XANTHATE IMPURITIES TO vAcuuM SYSTEM REFLUX OVERHEAD PROOucT VACUUM FRACTIONATOR CHARGE l 25 TRAYS REBO'LER QUANTITIES IN POUNDS PER DAY BOTTOMS BATCH FRACTIONATION 2 3 TIME OF OUT HR. 7 HRs. 15 HRS. TOP PREssuRE IOO MM. 50 MM. 5 MM.

BOTTOM TEMP. AT END OF CUT ISO-F 200F 350E TRAYS REQO. 25 25 IO INVENTORS C.F. BETHEA /2 L.W. POLLOCK giu hzru ATTORN EYS May 1, 1962 c. F. BETHEA ETAL 3,032,478

IMPROVED METHOD OF FRACTIONALLY DISTILLING LIQUIDS Filed July 15. 1955 4 Sheets-Sheet 4 OVERHEAD PRODUCT INVENTOR. C. F. BETHEA L.W. POLLOCK A TTORNE VS BOTTOMS United States Patent porationof Delaware Filed July 15, 1955, Ser. No. 522,194

4 Claims.

This invention relates to apparatus and method for fractionating liquids. In one aspect, this invention relates to a multi-purpose fractionator adjustably usable over a broad range of fractionation conditions. In another aspect, this invention rel-ates to fractionation apparatus containing vapor-liquid contact trays, sections of which are removable to provide for decreased pressure drop across the fractionation zone. In another aspect, this invention relates to damper means in operative communication with a fractionation zone to regulate vapor flow therein and control thereby downflow of liquid to reduce pressure drop across the fractionation zone. In still another aspect, this invention relates to a fractionating column containing tray means for receiving condensate in the fractionation zone, associated with supplemental reboiler means for reboiling the received condensate and returning vapor therefrom to the fractionation zone. In still another aspect, this invention relates to a fractionator equipped with grid-type trays, the upper surfaces of the members being positioned and shaped so as to collect condensate for withdrawal from the system. In still another aspect, this invention relates to a method for fractionating liquids in a fractionating zone under varying degrees of fractionation efficiency and vacuum.

In the utilization of fractionators for various purposes, various requirements of pressure, pressure drop, required separation, and plate efiiciency are encountered. Thus, a fractionator in order to be applicable to multi-purpose operations must be adaptable to rearrangement of its individual parts to meet the needs at hand.

An object of this invention is to provide fractionation apparatus and method. Another object is to provide means in a fractionation chamber, and method, for controlling pressure drop therein and for conducting fractionation over a broad range of separations and fractionation plate efliciencies. Other objects and aspects of this invention and its several related particulars are evident from a perusal of this disclosure, the drawings and the appended claims.

This invention is concerned with fractionation apparatus adaptable to operation under conditions of pressure drop, pressure, and plate efliciency over a broad range.

In accordance with this invention, a multi-purpose fractionating chamber is provided comprising an elongated shell; a plurality of vapor-liquid contact plates transversely disposed across said shell; means for supplying liquid to at least two of said plates, a section of at least one of said plates being removable therefrom; means for so removing such plate section from said plate; tray means in said chamber disposed to collect downwardly flowing liquid therein; means for supplying heat to a lower section of said chamber and reboiler means operatively connected with said tray means to receive liquid therefrom and to return resulting vapors to said chamber; and fluid inlet and outlet means connecting with said chamber. In accordance with other concepts of this invention, a fractionation chamber is provided containing at least one of the following elements: (1) at least one vapor-liquid contact tray or plate disposed across the chamber, containing a section which is adapted to be removed therefrom, and means for removing said section; (2) damper means in operative communication with the interior of the fractionation chamber for effectice ing control of vapor flow therethrough; (3) a tray assembly within the chamber disposed so as to collect downwardly flowing V operatively connected with the said tray assembly to receive liquid therefrom and to return resulting vapors to the said chamber; and (4) at least one vapor-liquid contact plate comprising a plurality of members to form a grid, each grid member having its upper surface shaped to collect liquid, and conduit means in communication with the said upper surface for conveying liquid therefrom to a point outside the chamber.

Still in accordance with this invention, improvements in a process for fractionating liquids are provided, comprising (l) dampening vapor flow through the zone'of fractionation in response to a predetermined increase in pressure drop across the said zone, whereby any condensate accumulating in the fractionation zone to cause an increase in pressure drop above the said predetermined value is caused to descend for withdrawal, with concomitant reduction in the said pressure drop; (2) accumulating condensate as a separate fraction in the zone of fractionation and withdrawing same from the said zone, reboiling withdrawn condensate and returning resulting vapors to the said zone; (3) removing a predetermined proportion of available vapor-liquid contacting surface in the fractionation zone to reduce pressure drop across the zone of fractionation in accordance with re-- quirements for decreased pressure drop'and decreased efficiency of fractionation that can be tolerated; and (4) passing vapors through a vapor-liquid contacting zone containing unobstructed and elongated passage-ways, whereby to achieve fractionation under decreased pressure drop conditions while at the same time collecting liquid on an upper portion of the said vapor-liquid contacting zone for withdrawal from the zone of fractionation.

Our invention is illustrated with reference to the at tached figures. FIGURE 1 illustrates a multi-purpose fractionating column equipped with damper means and multiple reflux feed lines for fractionating materials under different separation requirements, plate efliciency, and pressure drop conditions. FIGURE 2 shows a vaporliquid contact plate transversely disposed across an up right fractionation column, a section of the plate being adaptable to be removed or detached therefrom. FIG- URE 3 illustrates one form of damper construction of FIGURE 1. FIGURE 4 shows a tray assembly for collecting condensate formed in the fractionating column and serving as a kettle therefor, and connected with an external reboiler from which liquid and vapor is returned to the 'fractionating column. FIGURE 5 shows a fractionating column with multiple reflux feed lines and lines for withdrawal of condensate from various vapor-liquid contacting means. FIGURE 6 shows a sieve-type tray suitable for use as vapor-liquid contacting means of FIG- URE 5 having the upper surface of the members shaped to catch down-falling liquid in the column. FIGURE 7 shows a sectional view taken along the line 7-7 ofFIG- URE 6 of the tray element associated with means for withdrawal of liquid to points outside the column. FIG- URE 8 is illustrative of actuating means operatively connected with means in the said fractionating column for detaching a detachable plate section therein. FIGURE 9 shows a conventional bubble cap type fractionating column, by way of further illustration of apparatus of FIG- URE 2. FIGURES 10, 11 and I2 illustrate specific fractionation processes which require-pressure, pressure drop, separation, and plate etiiciency conditions over a broad range that can be conducted in a single vessel in accordance with this invention. FIGURE 13 illustrates a multipurpose fractionating column containing all. the elements described in the above FIGURES 1-9. Similar elements liquid therein and reboiler means 3 in FIGURE '13 are identified by the same referencenumerals as are used in FIGURES 1-9.

With reference to FIGURE 1, heating coil 9 is disposed in a bottom portion of upright chamber, or shell, 8, containing a plurality of vapor-liquid contacting trays, say about trays, such as trays 1 to 25 of which only, say, trays 1t 15, and 25 are shown. Associated with each of the latter trays are reflux inlet lines 52, 53, and 54. Chamber 8 is provided with feed conduit 27. Kettle product Withdrawal line 28 is connected with a lower portion of chamber 8 and overhead line 29 is connected with an upper portion thereof. Damper means 31, as illustrated in more detail with reference to FIGURE 3, is disposed in overhead line 29. A conventional commercial butterfly-type valve is a satisfactory damper means 31, but the use of any means for interrupting the flow of vapor in line 29 is within the scope of this invention.

With reference to FIGURE 2 is shown a vapor-liquid contacting tray, say tray 18, containing section 18' detachable therefrom, together with a detaching member such as rod 33 secured to section 18' and adapted to be raised or lowered such as by actuating means 20 outside chamber 18, see FIGURES 8 and 9.

With reference to FIGURE 4, tray assemblies such as donut type trays 35 and 36 are disposed in chamber 8 preferably fixed to the inner peripheral wall thereof to collect condensate in chamber 8 flowing down the side walls thereof and off of the top surface of deflecting means 35' and 36', the latter means adapted to prevent flow of liquid past the said trays. Conduits 37 and 38 are connected, respectively, with trays 35 and 36 to convey collected liquid therefrom to reboiler 9, from which vapor and liquid can be returned to chamber 8 as sprays 39 and 41 via lines 42 and 43, respectively. Overhead vapor line 29, cooler 46 therein and storage 47 are connected to a top portion of chamber 8 for vapor withdrawal and condensation. Overhead product withdrawal line 48 and reflux return line 54 are connected with storage 47 by way of line 51. Chamber 8 is provided with feed conduits 27 and 27'.

FIGURE 5 shows a plurality of reflux inlets 52, 53, and 54 to an upper portion of column 8, which contains vaporliquid contacting means, as for example, tray 56. Liquid condensate may be withdrawn from tray 56 through line 62 and delivered to the reflux entry conduit, as for example, through line 63 to line 52.

Sieve-type tray 56, shown with reference to FIGURE 6, is disposed substantially transversely across column 8 and comprises a plurality of elongated grid members 57 in substantially parallel and spaced relation with each other so as to form elongated unobstructed openings 58. Each member 57 has an upper cupped or dished surface 59, e.g., U-shaped so as to collect downwardly flowing or dropping liquid. Each of members 57, see FIGURE 7, is connected with liquid collector assembly 61, preferably a ring or trap disposed peripherally on the inner wall of chamber 8 and positioned so as to receive liquid flow from tray 56. Conduit 62 is connected with collector 61 and with lines 63, or to conduct flow of liquid from tray 56, advantageously for return of the liquid as reflux to the column.

With reference to FIGURE 8, rod 33 attached to detachable tray section 18' (see FIGURE 9) projects through packing gland 70 in the top of column 8. Cable 72 is attached to rod 33 by fastener 71 and is connected to winch 74 by pulley 73.

This invention provides in accordance with one embodiment, a fractionator, and its operation, containing all trays required for a given high plate requirement fractionation and also supplemental means for utilizing the same fractionating column for lower plate requirement separations without impairment in any way by elements present such as additional trays that are not needed in the operation at hand. Thus, We can maintain the column with, say, 25 bubble cap trays for certain high pressurehigh plate requirement fractionations and by' our invention use the same column for high vacuum separations ranging from flashing and condensation of vapors to efii-' cient high plate requirement vacuum separations, without development of high pressure drop ordinarily resulting from presence of unused trays, and without accumulation of liquid condensate on the unused trays to further add to the pressure drop.

Pressure drop across the contacting elements in the column has two components, (1) the pressure drop due to gas flow through the dry tray and (2) the pressure drop due to the liquid head on the tray. In a tray-type column in vacuum service, total pressure drop across the same is in the range of from 1 to 10 mm. of mercury per tray. Of this, about one-half is due to the liquid head and onehalf due to pressure drop across the dry tray. Thus, prior to this invention, when a fractionation employing a reduced number of trays, i.e., leaving dry trays in the column was carried out, the unused trays therein caused development of unnecessary pressure drop, thus impairing the efliciency of vacuum achieved and contributing toward damage to the heat-sensitive materials being processed. Further, liquid ordinarily collecting on the unused trays due to condensation caused additional head on the tray. These difliculties are, in accordance with our invention, substantially prevented.

Again referring to FIGURE 1, there is shown means whereby a 25-tray fractionator may be operated, with reflux returned to the tenth or fifteenth trays, the other trays, i.e., the tray sections between the eleventh and twentyfifth sections or between the sixteenth and twenty-fifth being unused, i.e., dry, and causing development of pressure drop across the column. However, as shown in FIGURE 2, the invention provides for trays in the fractionating column containing a detachable section, see 18' of FIGURE 2, which can be suspended in place to permit passage of vapors through the unused tray at substantially no pressure drop. This feature of the invention is illustrated in more detail with reference to FIGURES 8 and 9, the latter showing a conventional bubble-cap type column equipped with trays, each containing a central detachable section 18 whereby the unused trays shown are precluded from causing pressure drop across the column 8, and are moved in and out of position by way of detaching means 33 in operative connection with control means 20 of FIGURE 8.

Damper means of FIGURE 3 in operative communication with the interior of fractionator 8 decreases or preferably terminates fiow of vapors through line 29, andthus through chamber 8, to preclude accumulation of liquid on unused trays, i.e., the momentary stop of vapor flow permits liquid accumulated on the unused tray to drain therefrom for withdrawal. Thus, by dampening flow of vapors through fractionating column 8 in responseto pressure build up on the trays, accumulated liquid is caused to be dumped from the said tray for withdrawal and maintenance of the desired low pressure drop across the vessel. Upon removal of the accumulated liquid from the unused tray, normal pressure drop conditions are restored and dampening means of FIGURE 3, responsive thereto, is moved so as to permit normal vapor flow. Damper means 31 may be actuated manually in response to a measurement of pressure drop across the unused trays of the fractionator, but it is within the scope of this invention that the damper be actuated automatically controlled by a pressure controller of any type well known in the art. Thus, as shown in FIGURE 1, the pressure drop across the unused trays of the fractionator 8 can be measured by differential pressure transmitter and a signal representative thereof transmitted to pressure controller 81, which actuates damper means 31 in response to the pressure drop across the unused trays of the fractionator.

Tray assembly means of FIGURE 4, preferably a donut type tray, peripherally disposed in the column as shown with deflectormeans serves as a kettle, which with associated reboiler means 9' eliminates need for use of lower column portions and therefore precludes development of pressure drop that would otherwise be encountered across unused column trays. Thus, in the exemplary 25-tray column, above referred to, donut type trays 35 and 36 can advantageously be installed as the eleventh and sixteenth trays down from the top. Two additional trays can be added in the bottom of the column to give the required 25 function trays when needed. Liquid from either the eleventh or sixteenth tray (elements 35 or 36) can be pumped by forced circulation through the external reboiler 9' and sprayed back into the same tray whereby the donut tray serves as the kettle, and number of column trays can be adjusted depending on the use of the donut trays or internal reboiler as desired.

Sieve-type trays of FIGURES 6 and 7 serve to catch condensed liquid in non-pressure drop recessions there- 'on and discharge the collected liquid to points outside the column. Thus, sieve-type trays 56 can be employed for unused trays whereby to collect condensate passing down therefrom and to discharge same from the column, precluding thereby further accumulation of the said condensate with concomitantly increased pressure drop across the column. Advantageously, condensate collected and withdrawn from the column is recharged to column as reflux.

A multi-purpose fractionating column of this invention contains any one or all of the concepts illustrated with reference to the drawings and description herein. The said column may contain all of the elements as shown in FIGURE 13 or any combination, as desired, the feature of this invention being a column, and its operation, adaptable to be operated under pressure and theoretical plate requirement conditions over a broad range, thereby precluding need for maintaining a large number of separate columns for achieving these different purposes. Thus, as is evident, we have provided fractionating column structure that can be operated over varied tray requirement and pressure drop conditions thereby being particularly advantageous in the application to heat-sensitive materials under reduced energy requirements.

Exemplary of various type separations that can be made in a fractionating column in accordance with this invention although they are made under varied conditions of pressure, pressure drop, tray requirement, and efficiency, are those of FIGURES 10', 11 and 12. Thus, with reference to FIGURE 10, there is shown diagrammatically, a separation of polybutadiene from its solution in normal heptane, the heptane-polybutadiene comprising effluent from butadiene polymerization in presence of sodium and a hydrocarbon solvent, as set forth and claimed in U.S. Patent 2.631,l75 of Willie W. Crouch, Phillips Petroleum Company, issued March 10, 1953. The liquid polybutadiene must be separated from normal heptane and that can be done in a multi-purpose fractionator of this invention, there being only ten trays required in making the separation. With reference to FlG- URE 11, there is shown a vacuum separation of sodium ethyl xanthate from ethyl alcohol solution, in a process for the manufacture of di(ethylxanthogen)trisulfide, the sodium ethyl xanthate having previously been prepared by reaction of ethanol and caustic soda with carbon disulfide. The said process for manufacture of di(ethylxanthogen) trisulfide is disclosed and claimed in the copending application of C. W. Kruse and C. W. Osborn, Serial No. 388,684, filed October 27, 1953. In this separation, as shown in the data of FIGURE 11, about 15 trays are required. With reference to FIGURE 12, there is shown a batch fractionation used in the purification of reaction product of butadiene with furfural produced in accordance with process disclosed and claimed in US. Patent 2,683,151 of John C. Hillyer and Daniel A. Nicewander, Phillips Petroleum Company, issued July 6, 1954. As shown in the data of FIGURE 12, which is a batch fractionation, three cuts are made, the first being rich in butadiene dimer, the second rich in furfural and the third rich in butadiene-furfural cotrimer product. Cuts '6 1 and 2 require 25 trays but for cut 3, only 10 trays are needed. A material balance for the batch fractionation is shown in Table I, quantities being expressed in pounds per day.

T able 1 Charge Cut 1 Cut; 2 Cut 3 Residue Butadiene t Butadiene Dimer; 2, 404 2, 290 114 Thus, with reference to the foregoing described process embodiments, and by way of further illustration of this invention, a conventional 25-tray fractionator column will be required for fractionation of a cut of a material such as cut 1, whereas a material such as cut 3 can be fractionated in a 10-tray assembly. Thus, a cut 3 could be charged onto a donut tray as shown in FIGURE 4 as tray 15, with the remainder of the column out of use, and unused trays therein being of no impairment to fractionation of cut 3. However, if desired, cut 3 could be introduced to the column as shown in FIGURE 9 with trays 11 to 25 equipped with detachable sections, sus-' pended as shown, to reduce pressure drop across the fractionation zone.

A sieve-type tray of FIGURE 6 can be advantageously employed when fractionating materials such as cut 3 above referred to, or an ethanol-sodium ethyl xanthate mixture (FIGURE 11) whereby to entrap condensate formed-due to heat losses in the upper column portion and remove the same without its accumulation. Unobstructed passageways in such sieve-type element provide for free 0 flow of vapors therethrough with substantially no pressure drop resulting therefrom.

Reasonable variation and modification are possible within the scope of the foregoing disclosure, the drawings, and the appended claims to the invention, the essence of which is that (1) means and method has been provided in a fractionation system, for controlling pressure drop therein and for fractionation over a broad range of fractionation plate requirements, said means comprising at least one of (a) at least one vapor-liquid contact tray or plate disposed across the chamber, containing a section which is adapted to be removed therefrom, and means for removing said section; (b) damper means in operative communication with the interior of the fractionation chamber for etfecting control of vapor flow therethrough; (c) a tray assembly within the chamber disposed so as to collect downwardly flowing liquid therein and reboiler means operatively connected with the said tray assembly to receive liquid therefrom and to return resulting vapors to the said chamber; and (d) at least one vapor-liquid contact plate comprising a plurality of members to form agrid, each grid member having its upper surface shaped to collect liquid, and conduit means in communication with the said upper surface for conveying liquid therefrom to a point outside the chamber; and (2) in accordance with another concept a multi-purpose fractionating column and its operation has been provided which can be utilized for high efficiency fractionati'ons and which is adaptable to use in carrying out relatively low plate requirement fractionations while maintaining at least one of the trays unused, without development of pressure drop across the unused trays.

We claim:

1. A method of fractionating a fluid in a fractionating column having disposed therein a plurality of liquidcollecting tray assemblies, the number of said tray assemblies being greater than that required to effect a desired fractionation of said fluid and the excess tray assemblies causing excessive pressure drop in said fractionating column, which comprises introducing fluid feed into said column, withdrawing vapor from the upper part 7 of said column, returning reflux liquid to the upper part of said column, utilizing a first liquid-collecting tray assembly as the effective kettle of said column into which is supplied the only heat supplied to said column other than contained in the feed to said column, said liquidcollecting tray assembly being disposed in said column below the level at which said feed is introduced and above at least a second liquid-collecting tray assembly, said second liquid-collecting tray assembly being one of said excess tray assemblies causing said excessive pressure drop, withdrawing a substantial proportion of liquid from said first liquid-collecting tray assembly and from said column, reboiling withdrawn liquid, and returning the resulting vapor to said column at a level above said first liquid-collecting tray assembly, allowing unvaporized liquid on said first liquid-collecting tray assembly to flow from said first liquid-collecting tray assembly downwardly in said column through said excessive tray assemblieswhich are below said first liquid-collecting tray assembly and are, therefore, at a substantially lower temperature, thus substantially eliminating the fractionating function of these liquid-collecting tray assemblies below said first liquid-collecting tray assembly and thereby preventing pressure drop in said column below said first liquidcollecting tray assembly, and withdrawing from said column all of the liquid passing downwardly through said excess tray assemblies.

2. A method of fractionating a fluid in a fractionating column having disposed therein a plurality of liquidvapor contacting tray assemblies, the number of said tray assemblies being greater than that required to effect a desired fractionation of said fluid and the excess tray assemblies causing excessive pressure drop in said fractionating column, which comprises introducing fluid feed into said column at an intermediate level therein adjacent a first series of tray assemblies desired to be used in the fractionation of said fluid, said first series of tray assemblies being disposed in said column below at least a second series of tray assemblies, said second series of tray assemblies being excess tray assemblies causing said excessive pressure drop, withdrawing vapor from the upper part of said column, condensing said vapor, returning at least a part of the thus-condensed vapor to said column at a level adjacent said first series of tray assemblies, determining the pressure drop across said second series of tray assemblies, and momentarily terminating said withdrawing of vapor from the upper part of said column in response to a predetermined increase in said pressure drop whereby liquid accumulating on said second series of tray assemblies disposed in said column above said first series of tray assemblies is caused to descend therefrom, thereby eliminating the fractionating function of said second series of tray assemblies and reducing the pressure drop across the same.

3. A method of fractionating a fluid in a fractionating column having disposed therein a plurality of liquid-collecting tray assemblies, the number of said tray assemblies being greater than that required to effect a desired fractionation of said fluid and the excess tray assemblies causing excessive pressure drop in said fractionating column which comprises introducing fluid feed into said column at an intermediate level therein adjacent a first series of tray assemblies desired to be used in the fractionation of said fluid, said first series of tray assemblies being disposed in said column below a second series of tray assemblies, said second series of tray assemblies being excess tray assemblies causing said excessive pressure drop, said second series of tray assemblies having removable sections, withdrawing vapor from the upper part of said column, condensing said vapor, returning at least a part of the thus-condensed vapor to said column at a point adjacent said first series of tray assemblies, and maintaining said removable sections of said second series of tray assemblies in a raised position during the fractionating or" said fluid so as to decrease the pressure drop across said second series of tray assemblies and achieve the desired fractionation of said fluid in said fractionating column.

4. A method of fractionating a fluid in a fractionating column having disposed therein a plurality of liquid-collecting tray assemblies, the number of said tray assemblies being greater than that required to effect a desired fractionation of said fluid and the excess tray assemblies causing excessive pressure drop in said fractionating column, which comprises introducing fluid feed into said column at an intermediate level therein adjacent a first series of tray assemblies desired to be used in the fractionation of said fluid, said first series of tray assemblies being disposed in said column below at least a second series of tray assemblies, said second series of tray assemblies being excess tray assemblies causing said excessive pressure drop, withdrawing vapor from the upper part of said column, condensing said vapor, returning at least a part of the thus-condensed vapor to said column at a level adjacent said first series of tray assemblies, withdrawing substantially all the liquid from said second series of tray assemblies and from said column so as to reduce the pressure drop across said second series of tray assemblies, and returning the thus-withdrawn liquid to said column at a level adjacent said first series of tray assemb ies.

References Cited in the file of this patent UNITED STATES PATENTS 79,260 Savalle June 23, 1868 160,951 Prentiss Mar. 16, 1875 1,053,349 Blauvelt Feb. 18, 1913 1,560,137 Bernard Nov. 3, 1925 1,670,743 Schnable May 22, 1928 1,848,462 Corbett Mar. 8, 1932 1,865,400 McConnell June 28, 1932 2,108,659 Dunham Feb. 15, 1938 2,113,130 Dunham Apr. 5, 1938 2,206,507 Kuhni July 2, 1940 2,218,993 Rupp Oct. 22, 1940 2,237,271 Dunham Apr. 1, 1941 2,266,359 Edwards Dec. 16, 1941 2,290,055 Kinsey July 14, 1942 2,357,710 Ulbrich Sept. 5, 1944 2,456,398 Gerhold Dec. 14, 1948 2,534,870 Kraft Dec. 19, 1950 2,575,186 Ryant Nov. 13, 1951 2,581,051 Smith Jan. 1, 1952 2,611,596 Glitsch Sept. 23, 1952 2,614,971 Burton Oct. 21, 1952 FOREIGN PATENTS 4,464 Great Britain of 1885 4,869 Great Britain of 1889 16,806 Great Britain of 1887 

1. A METHOD OF FRACTIONATING A FLUID IN A FRACTIONATING COLUMN HAVING DISPOSED THEREIN A PLURALITY OF LIQUIDCOLLECTING TRAY ASSEMBLIES, THE NUMBER OF SAID TRAY ASSEMBLIES BEING GREATER THAN THAT REQUIRED TO EFFECT A DESIRED FRACTIONATION OF SAID FLUID AND THE EXCESS TRAY ASSEMBLIES CAUSING EXCESSIVE PRESSURE DROP IN SAID FRACTIONATING COLUMN, WHICH COMPRISES INTRODUCING FLUID FEED INTO SAID COLUMN, WITHDRAWING VAPOR FROM THE UPPER PART OF SAID COLUMN, RETURNING REFLUX LIQUID TO THE UPPER PART OF SAID COLUMN, UTILIZING A FIRST LIQUID-COLLECTING TRAY ASSEMBLY AS THE EFFECTIVE KETTLE OF SAID COLUMN INTO WHICH IS SUPPLIED THE ONLY HEAT SUPPLIED TO SAID COLUMN OTHER THAN CONTAINED IN THE FEED TO SAID COLUMN, SAID LIQUIDCOLLECTING TRAY ASSEMBLY BEING DISPOSED IN SAID COLUMN BELOW THE LEVEL AT WHICH SAID FEED IS INTRODUCED AND ABOVE AT LEAST A SECOND LIQUID-COLLECTING TRAY ASSEMBLY, SAID SECOND LIQUID-COLLECTING TRAY ASSEMBLY BEING ONE OF SAID EXCESS TRAY ASSEMBLIES CAUSING SAID EXCESSIVE PRESSURE DROP, WITHDRAWING A SUBSTANTIAL PROPORTION OF LIQUID FROM SAID FIRST LIQUID-COLLECTING TRAY ASSEMBLY AND FROM SAID COLUMN, REBOILING WITHDRAWN LIQUID, AND RETURNING THE RESULTING VAPOR TO SAID COLUMN AT A LEVEL ABOVE SAID FIRST LIQUID-COLLECTING TRAY ASSEMBLY, ALLOWING UNVAPORIZED LIQUID ON SAID FIRST LIQUID-COLLECTING TRAY ASSEMBLY TO FLOW FROM SAID FIRST LIQUID-COLLECTING TRAY ASSEMBLY DOWNWARDLY IN SAID COLUMN THROUGH SAID EXCESSIVE TRAY ASSEMBLIES WHICH ARE BELOW SAID FIRST LIQUID-COLLECTING TRAY ASSEMBLY AND ARE, THEREFORE, AT A SUBSTANTIALLY LOWER TEMPERATURE, THUS SUBSTANTIALLY ELIMINATING THE FRACTIONATING FUNCTION OF THESE LIQUID-COLLECTING TRAY ASSEMBLIES BELOW SAID FIRST LIQUID-COLLECTING TRAY ASSEMBLY AND THEREBY PREVENTING PRESSURE DROP IN SAID COLUMN BELOW SAID FIRST LIQUIDCOLLECTING TRAY ASSEMBLY, AND WITHDRAWING FROM SAID COLUMN ALL OF THE LIQUID PASSING DOWNWARDLY THROUGH SAID EXCESS TRAY ASSEMBLIES. 